How can I fix a wrinkle?
After filling you might want to triangulate faces CtrlT). Fix the topology by moving vertices. Then you can dissolve unwanted faces (press Space, write dissolve, select "dissolve faces"). 

Another way (the easier and faster way) is to build an extra cube, and with the boolean modifier, you perfectly cut the face of your model by the face of that extra cube. 

To fix the current topology, you might also try to "mark sharp" some of the edges, and or, to bevel or create extra loopcuts near areas you want to be sharper. That way the subdivision modifier won't round up as much on those areas.